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 #include &lt;QPainter&gt;

 #include &quot;edge.h&quot;
 #include &quot;node.h&quot;

 #include &lt;math.h&gt;

 static const double Pi = 3.14159265358979323846264338327950288419717;
 static double TwoPi = 2.0 * Pi;

 Edge::Edge(Node *sourceNode, Node *destNode)
     : arrowSize(10)
 {
     setAcceptedMouseButtons(0);
     source = sourceNode;
     dest = destNode;
     source-&gt;addEdge(this);
     dest-&gt;addEdge(this);
     adjust();
 }

 Edge::~Edge()
 {
 }

 Node *Edge::sourceNode() const
 {
     return source;
 }

 void Edge::setSourceNode(Node *node)
 {
     source = node;
     adjust();
 }

 Node *Edge::destNode() const
 {
     return dest;
 }

 void Edge::setDestNode(Node *node)
 {
     dest = node;
     adjust();
 }

 void Edge::adjust()
 {
     if (!source || !dest)
         return;

     QLineF line(mapFromItem(source, 0, 0), mapFromItem(dest, 0, 0));
     qreal length = line.length();
     QPointF edgeOffset((line.dx() * 10) / length, (line.dy() * 10) / length);

     prepareGeometryChange();
     sourcePoint = line.p1() + edgeOffset;
     destPoint = line.p2() - edgeOffset;
 }

 QRectF Edge::boundingRect() const
 {
     if (!source || !dest)
         return QRectF();

     qreal penWidth = 1;
     qreal extra = (penWidth + arrowSize) / 2.0;

     return QRectF(sourcePoint, QSizeF(destPoint.x() - sourcePoint.x(),
                                       destPoint.y() - sourcePoint.y()))
         .normalized()
         .adjusted(-extra, -extra, extra, extra);
 }

 void Edge::paint(QPainter *painter, const QStyleOptionGraphicsItem *, QWidget *)
 {
     if (!source || !dest)
         return;

     <span class="comment">//</span> Draw the line itself
     QLineF line(sourcePoint, destPoint);
     painter-&gt;setPen(QPen(Qt::black, 1, Qt::SolidLine, Qt::RoundCap, Qt::RoundJoin));
     painter-&gt;drawLine(line);

     <span class="comment">//</span> Draw the arrows if there's enough room
     double angle = ::acos(line.dx() / line.length());
     if (line.dy() &gt;= 0)
         angle = TwoPi - angle;

     QPointF sourceArrowP1 = sourcePoint + QPointF(sin(angle + Pi / 3) * arrowSize,
                                                   cos(angle + Pi / 3) * arrowSize);
     QPointF sourceArrowP2 = sourcePoint + QPointF(sin(angle + Pi - Pi / 3) * arrowSize,
                                                   cos(angle + Pi - Pi / 3) * arrowSize);
     QPointF destArrowP1 = destPoint + QPointF(sin(angle - Pi / 3) * arrowSize,
                                               cos(angle - Pi / 3) * arrowSize);
     QPointF destArrowP2 = destPoint + QPointF(sin(angle - Pi + Pi / 3) * arrowSize,
                                               cos(angle - Pi + Pi / 3) * arrowSize);

     painter-&gt;setBrush(Qt::black);
     painter-&gt;drawPolygon(QPolygonF() &lt;&lt; line.p1() &lt;&lt; sourceArrowP1 &lt;&lt; sourceArrowP2);
     painter-&gt;drawPolygon(QPolygonF() &lt;&lt; line.p2() &lt;&lt; destArrowP1 &lt;&lt; destArrowP2);
 }</pre>
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